Not really a function.  Returns the value of the last command in the
sequence of commands indicated by BLOCK.  When modified by the while
 or
until
 loop modifier, executes the BLOCK once before testing the loop
condition.  (On other statements the loop modifiers test the conditional
first.)
do BLOCK
 does not count as a loop, so the loop control statements
next, last, or
redo cannot be used to leave or restart the block.
See perlsyn for alternative strategies.
Uses the value of EXPR as a filename and executes the contents of the file as a Perl script:
do './stat.pl'
 is largely like
- eval `cat stat.pl`;
except that it's more concise, runs no external processes, and keeps
track of the current filename for error messages. It also differs in that
code evaluated with do FILE
 cannot see lexicals in the enclosing
scope; eval STRING
 does.  It's the same, however, in that it does
reparse the file every time you call it, so you probably don't want
to do this inside a loop.
Using do with a relative path (except for ./ and ../), like
- do 'foo/stat.pl';
will search the @INC  directories, and update
%INC  if the file is found.  See @INC in perlvar
and %INC in perlvar for these variables. In particular, note that
whilst historically @INC  contained '.' (the
current directory) making these two cases equivalent, that is no
longer necessarily the case, as '.' is not included in @INC
 by default
in perl versions 5.26.0 onwards. Instead, perl will now warn:
If do can read the file but cannot compile it, it returns undef and sets an error message in $@ . If do cannot read the file, it returns undef and sets $! to the error. Always check $@ first, as compilation could fail in a way that also sets $! . If the file is successfully compiled, do returns the value of the last expression evaluated.
Inclusion of library modules is better done with the use and require operators, which also do automatic error checking and raise an exception if there's a problem.
You might like to use do to read in a program configuration file. Manual error checking can be done this way:
- # Read in config files: system first, then user.
- # Beware of using relative pathnames here.
- for $file ("/share/prog/defaults.rc",
- "$ENV{HOME}/.someprogrc")
- {
- unless ($return = do $file) {
- warn "couldn't parse $file: $@" if $@;
- warn "couldn't do $file: $!" unless defined $return;
- warn "couldn't run $file" unless $return;
- }
- }