use strict;
use warnings;

my @extensions = qw( 123 456 );
my $extlist = join('|', @extensions);
my $re = qr/some_text=".*\.(?:$extlist)"/;

while(<DATA>){
  chomp;
  if ($_ =~/$re/i) {
    print "$_ MATCHES\n";
  }else{
    print "$_ does not match\n";
  }
}

__DATA__
some_text="something.123"
some_text="asdf.456"
some_text="asdf.457"

##</code><code>##

some_text="something.123" MATCHES
some_text="asdf.456" MATCHES
some_text="asdf.457" does not match