#!/usr/bin/perl -w
use strict;

my $EXON_IN;
for (my $i=0; $i < 5; $i++){
    $EXON_IN = <$i/*test.txt>;
    print "$i, -$EXON_IN-\n";
}

##</code><code>##

[scott@blast test]$ ./test.pl
0, -0/0_test.txt-
Use of uninitialized value in concatenation (.) or string at ./test.pl line 7.
1, --
2, -2/2_test.txt-
Use of uninitialized value in concatenation (.) or string at ./test.pl line 7.
3, --
4, -4/4_test.txt-

##</code><code>##

[scott@blast test]$ ls -R
.:
0  1  2  3  4  test.pl

./0:
0_test.txt

./1:
1_test.txt

./2:
2_test.txt

./3:
3_test.txt

./4:
4_test.txt