How are you printing it out?

print $f,$l,"\n";
#becomes one of:
{ local $, = ' '; print $f, $l; print "\n"; }
print $f, ' ', $l, "\n";
print join(' ', $f, $l), "\n";
[download]

or maybe

print $f . $l . "\n";
# becomes
print $f . " " . $l . "\n";
[download]

or maybe

print "$f$l\n";
# becomes
print "$f $l\n";
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or maybe

printf "%s%s\n", $f, $l;
# becomes
printf "%s %s\n", $f, $l;
[download]

But I'm not sure what you're doing now, so it's hard to tell which one is most similar to your existing code.