use strict;
use warnings;

my $in_file_name = 'sample.txt';

{
    local $/ = '.slow';
    open my $input, '<', $in_file_name or die $!;
    while( my $rec = <$input> ) {
        chomp $rec;
        next unless $rec =~ m/(\d+)\.pace(.+)$/s;
        my( $out_file_name, $capture ) = ( $1, $2 );
        open my $output, '>', $out_file_name or die $!;
        print $output $capture;
        close $output or die $!;
    }
    close $input;
}