imperl, I suspect you did not read this, which suggests you to print out the filename you're building: print is your friend in debugging! Moreover, use strict and use warnings should never miss when building example codes to post here: they will answer many questions much more quickly than us! As a final note, you should use <code> tags when posting :)

I think that you have problems with the newline: when you read from STDIN, it will be put inside $f. Just chomp inputs:

print "Enter the common character in the batch file:\n";
chomp(my $f = <STDIN>);
print "Enter the number in the last batch file:\n";
chomp(my $l=<STDIN>);
# Now both $f and $l have no trailing newline
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