use strict;
use warnings;
use feature 'say';

my $text = do{ local $/; <DATA> };
my @pmids = $text =~ /PMID:\s+(\d+)/gm;
say for @pmids;

__DATA__
 ---------
EFETCH RESULT(1..3): [
Some title
some author

Author information: 
blah blah blah
PMID: 24590727 

 ---------
EFETCH RESULT(1..3): [
Some title
some author

Author information: 
blah blah blah
PMID: 45867737 

 ---------
EFETCH RESULT(1..3): [
Some title
some author

Author information: 
blah blah blah
PMID: 52497072