You must be careful not to match lines that are not consecutive.

use strict;
use warnings;
my $string =
     "foo LAST\n"
    ." blftz\n"
    ."LINE fum\n"
    ;
print "MATCH\n" if ($string =~ m/LAST$ .* ^LINE/xms) ;
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You could solve this with:

print "MATCH\n" if ($string =~ m/LAST$ ..? ^LINE/xms) ;
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