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perl -le '$patt = shift; print qr/$patt/' 'a\Ubc'
# prints (?-xism:a\Ubc)

perl -le '$patt = shift; print qr/$patt/' 'a\\Ubc'
# prints (?-xism:a\\Ubc)

my $patt = shift; # e.g. 'a\Ubc' from the shell
$patt = qr/$patt/;
my $target = 'aBC';
print $target =~ /$patt/; # prints nothing

my $patt = shift;
$patt = eval "qr/$patt/"; # eeeww
# patt is now (?-xism:aBC) if given 'a\Ubc'
my $target = 'aBC';
print $target =~ /$patt/; # prints 1