Build up your regular expression programatically, and use the (?i-xsm) or (?-xism) constructs (documented in perlre).

In other words....

my $insensitive = '(?i-xsm)';
my $sensitive   = '(?-xism)';

my $target = "Abcde";

foreach my $re_switch ( $insensitive, $sensitive ) {
    if( $target =~ m/$re_switch([a-z])/ ) {
        print "$1\n";
    }
}
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And the output...

A
b
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