use warnings;
use strict;
use Data::Dumper qw(Dumper);

my @clk =  ('ux_prim_clk', 'ux_side_clk', 'ux_xtal_frm_refclk');

my @clk_output = map [ split /_/, $_ ], @clk;

print Dumper \@clk_output;

my @clk_new = map{my $line =join ("_", @$_); $line}@clk_output;

print Dumper \@clk_new;