I'm not sure this has anything to do with Test::More.

use Test::More 'tests' => 7;  # all tests pass

my $string = "abc\ndef\nghi";
my $pattern = qr/def/;
my $pattern_m = qr/^def$/m;

ok( $string =~ /^def$/m, 'literal/m pattern matches' );
ok( $string =~ $pattern, 'plain qr stored pattern matches' );

is( "$pattern_m", '(?m-xis:^def$)', 'qr/m as string: (?m-xis:^def$)');
like( $string, '/(?m-xis:^def$)/',  'string (?m-xis:^def$) matches' );
ok( $string =~ /(?m-xis:^def$)/,    'literal (?m-xis:^def$) matches');

ok( ! ($string =~ $pattern_m), 'qr/m stored pattern does not match' );
unlike( $string, $pattern_m,   'qr/m stored pattern not "like"' );
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Perhaps I've done something foolish here (using Test::More to check Test::More, maybe), but this looks like a bug.

Update after gary.monson's reply: Sorry I wasn't clear. This looks to me like a bug in perl. Without running it, I would have expected the last two tests to come out the opposite of how they did. I originally thought the OP's behavior might have to do with how Test::Builder uses string eval to do its comparison, but that doesn't seem to be the case.