You don't need the foreach loop as you have for @lines.

#!/usr/bin/perl -w
use strict;
my $SCU = 'C:/Users/Desktop/a.txt';
open (FILE, '<', $SCU) or die "$SCU File not found : $!";
my @lines = <FILE>;
close (FILE);

my @newlines;
#foreach(@lines) {
   push @newlines, s/test/xyz/rg for @lines;
#}

open (FILE, '>', $SCU) or die "Could not open file $SCU: $!";
print FILE @newlines;
close (FILE);
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poj