$hash{/abc_(.*)_ghi/} = boom_$1

##</code><code>##

use Data::Dumper;

my %hash = (
            xxx_text10_yyy => 1,
            xxx_text11_yyy => 1,
            mmm_text12_nnn => 1,
            mmm_text13_nnn => 1,
            abc_text6_ghi  => 1,
            abc_text7_ghi  => 1,
            abc_text8_ghi  => 1,
            abc_text9_ghi  => 1
            );

print Dumper(\%hash);

foreach my $key (keys %hash)
{
$hash{$key}    = "boom_$1" if ($key =~ /abc_(.*)_ghi/); 
$hash{$key}    = "doom_$1" if ($key =~ /mmm_(.*)_nnn/); 
}

print Dumper(\%hash);