Assuming that you have the strings in an array and want the strings that match "_theWord" to be placed in another array, this should work.

use strict;
use warnings;

my @strings = qw{
   ac_dajhta_theWord
   sd_dfdfe_fdfdv
   ca_hereeds_theWord
   se_dcvdfdef_dsfd
   cx_hserj_theWord
   fa_dferefvdvv_fadfde};

my @matchingStrings = grep m{_theWord}, @strings;

print qq{$_\n} for @matchingStrings;
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The output is

ac_dajhta_theWord
ca_hereeds_theWord
cx_hserj_theWord
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I hope this is of use.

Cheers,

JohnGG

Update: corrected wonky <code> tag