This is one way to accomplish the task:

#!/usr/bin/perl -w
#
#
my @data = (7, 314, 23, 513465, 4444, 29 );

for (@data) {
    print " Num: ", six_zeroes($_), "\n";
}

sub six_zeroes {
    my $num = shift;
    return substr("000000" . $num, -6 );
}
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And the results are:

~/perl/perlmonks$ ./leading_zeroes.pl
 Num: 000007
 Num: 000314
 Num: 000023
 Num: 513465
 Num: 004444
 Num: 000029
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