#!/usr/bin/perl
use strict;
use warnings;

my $string = 'test';
my $inputs = 'test.txt';

# see perlfunc
open( my $in, '<', $inputs) or die "can't open $inputs.\n";

while ( <$in> ) { # see perlsyn, perlop
    if ( $_ =~ m/^$string(.*)/ ) { # see perlre
        print $1; # see perlvar, perlre
    }
}
close( $in ); # see perlfunc