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# do

Perl 5 version 22.0 documentation
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# do

Perl functions A-Z | Perl functions by category | The 'perlfunc' manpage

- do BLOCK

  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.

- do EXPR

  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, keeps track of the current filename for error messages, searches the `@INC` directories, and updates `%INC` if the file is found. See @INC in perlvar and %INC in perlvar for these variables. It also differs in that code evaluated with `do FILENAME` 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.

  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    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;        }    }