use 5.010;
use strict;
use warnings;
 
my @a=('abc',1);
given (@a) {
    when (/abc/  ) { print '/abc/'."\n" ;continue}
    when ("abc"  ) { print "abc\n" ;continue}
    #when (1      ) { print "#\n"   ;continue}
    when (['abc',1]) { print "copy\n" ;continue}
    when (\@a    ) { print "self\n" }
}
 
print "as it should be: ",@a~~/abc/;
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OUTPUT

abc
copy
self
as it should be 1
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PS: I have to go, too ...

You need to do when (qr/abc/). when(/abc/) appears to do when($_ ~~ ($_ =~ /abc/)).

use 5.010;
use strict;
use warnings;

my @a=qw(abc def);

given (@a) {
    when (/abc/  ) { say '/abc/';   continue; }
    when (qr/abc/) { say 'qr/abc/'; continue; }
}

say '--';

if (@a ~~ /abc/  ) { say '/abc/';   }
if (@a ~~ qr/abc/) { say 'qr/abc/'; }
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qr/abc/
--
/abc/
qr/abc/
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The discrepency is odd, though.

But it is documented.

But when EXPR is one of the below exceptional cases, it is used directly as a boolean:

• [...]
• a regular expression match, i.e. /REGEX/ or $foo =~ /REGEX/, or a negated regular expression match (!/REGEX/ or $foo !~ /REGEX/).
• [...]

It makes sense. It allows

given ($x) {
   when (/abc/) { ... }
   when (/def/) { ... }
   when (/ghi/) { ... }
}
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to mean

if    ($x =~ /abc/) { ... }
elsif ($x =~ /def/) { ... }
elsif ($x =~ /ghi/) { ... }
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