My suggestion ( tested)

my $infile = "try.txt";
open my $INFILE, q{<}, $infile or die;

my $outfile = "results.txt";
open my $OUTFILE, q{>}, $outfile or die; 

while (my $line = <$INFILE>){

$line =~ s/\s+$//;
print STDOUT "the line is $line\n";
#if ($line =~ /(\d+):(\d)::\/(\d+)/){
    if ($line =~ /(\d+):(.*)\/(\d+)/){
        print $OUTFILE "the first decimals are $1\n";
        print $OUTFILE "the last decimals are $3\n";
        print $OUTFILE "the middle element is $2\n";
        
        
    }
print $OUTFILE "\n";    
}

close $INFILE;
close $OUTFILE;
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In $1: you have 2001

Perl understands the ":" in the regex

in $2, you have 1:: or 0:6:4003:: or :12

in $3, you have 64 or 128

Here are the reuslts

the first decimals are 2001
the last decimals are 64
the middle element is 1::

the first decimals are 2001
the last decimals are 64
the middle element is 0:6:4003::

the first decimals are 2001
the last decimals are 128
the middle element is :12
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