You could turn the problem inside out: load the test values into memory then scan the large reference file one line at a time to perform the matching:
#!/usr/bin/perl
use strict;
my $reps = <<REPS;
chr1 100 120 feature1
chr1 200 250 feature2
chr2 150 200 feature1
chr2 280 350 feature1
chr3 100 150 feature2
chr3 300 450 feature2
REPS
my %tests;
while (my $line = <DATA>) {
$line =~ s/[\n\r]//g;
my @array = split /\s+/, $line;
$tests{$array[0]}{$array[1]}{'end'} = $array[2];
$tests{$array[0]}{$array[1]}{'rep'} = $array[3];
}
open my $repIn, '<', \$reps;
while (<$repIn>) {
my ($chr, $start, $end, $rep) = split ' ';
next if !exists $tests{$chr};
for my $s (keys %{$tests{$chr}}) {
if ($start <= $tests{$chr}{$s}{'end'}) {
last if $s >= $end;
print "$chr $start $end $rep\n";
}
}
}
__DATA__
chr2 160 210
True laziness is hard work