This assumes that the input filename contains at least one dot. The number will precede the last dot in the input:

#! perl -sw
use strict;

my $file = $ARGV[0];
my( $prefix, $suffix ) = ( $file =~ m[^(.+)\.([^.]+)] );

open I, '<', $file or die $!;

my @outs;
open $outs[ $_ ], '>', "$prefix.${_}.$suffix" or die $! for 1 .. 4;

my $out = 0;

while( <I> ) {
    print { $outs[ $out+1 ] } $_;
    if( /^5/ ) {
         ++$out;
         $out %= 4;
    }
}
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