use strict;
use warnings;

my %substitutions = ();
while ( <DATA> )
{
    chomp;
    my ($pattern, $replace) = split;
    my $rxPattern = qr{(?si)$pattern};
    $substitutions{$rxPattern} = $replace;
}

my $str = q{a capital M and Fish};
print qq{$str\n};

$str =~ s{$_}{$substitutions{$_}}
   for keys %substitutions;

print qq{$str\n};

__END__
\s*M\s* MALE
\s*F\s* FEMALE