In this snippet it looks like the /m modifier does not modify a regex if it came from a pre-compiled regex. (4th print is 0). If I am not mistaken this was not always like this. I think in 5.8.x the 4th print still printed 1. Can you confirm this? Can someone point me to the explanation of this? 
use strict;
use warnings;

my $str = "x\nab\ny";
my $re = '^ab$';
    
print $str =~ /$re/ ? 1 : 0, "\n";    # 0
print $str =~ /$re/m ? 1 : 0, "\n";   # 1
    
my $qre = qr/^ab$/;
print $str =~ /$qre/ ? 1 : 0, "\n";   # 0
print $str =~ /$qre/m ? 1 : 0, "\n";  # 0  but I think 5.8.x still had
+ this as 1

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Update

Thanks for all the responses and links!

In reply to This regex surprised me by szabgab

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