$ perl -wE 'use Data::Dump; my %h; dd \%h; $_ = "a_b_c"; $h{/a_(.*)_c/} = boom_$1; dd \%h'
{}
Can't call method "boom_" on an undefined value at -e line 1.

##</code><code>##

$ perl -wE 'use Data::Dump; my %h; dd \%h; $_ = "a_b_c"; $h{/a_(.*)_c/} = "boom_$1"; dd \%h'
{}
Use of uninitialized value $1 in concatenation (.) or string at -e line 1.
{ 1 => "boom_" }

##</code><code>##

$ perl -wE 'use Data::Dump; my %h; dd \%h; $_ = "a_b_c"; $h{(/a_(.*)_c/)[0]} = "boom_$1"; dd \%h'
{}
Use of uninitialized value $1 in concatenation (.) or string at -e line 1.
{ b => "boom_" }

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;

use Data::Dump;

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
);  

my %xref = (
    abc => 'boom',
    mmm => 'doom',
    xxx => 'zoom',
);

my $re = qr{(?x: ^ ( [^_]+ ) _ ( [^_]+ ) )};

/$re/ and $hash{$_} = join '_', $xref{$1}, $2 for keys %hash;

dd \%hash;

##</code><code>##

{
  abc_text6_ghi  => "boom_text6",
  abc_text7_ghi  => "boom_text7",
  abc_text8_ghi  => "boom_text8",
  abc_text9_ghi  => "boom_text9",
  mmm_text12_nnn => "doom_text12",
  mmm_text13_nnn => "doom_text13",
  xxx_text10_yyy => "zoom_text10",
  xxx_text11_yyy => "zoom_text11",
}