my $string = "(\d+)\:(\d+) (\d+)\/(\d+)\/(\d+)";

What is the purpose of this line? Does it really help you to do what you want to? What happens if you leave it out entirely?

s/// works in place (that is, on the variable it is bound to). What happens if you rewrite it as this:

if ($_ =~ m/(\d+)\:(\d+) (\d+)\/(\d+)\/(\d+)/ ){
    $_ =~ s/ /     /;
}
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And of course, m// and s/// works on $_ in lieu of any other variable, so you could even just go:

if (m/(\d+)\:(\d+) (\d+)\/(\d+)\/(\d+)/ ){
    s/ /     /;
}
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Which could be shortend to:

s/ /   / if  m/(\d+)\:(\d+) (\d+)\/(\d+)\/(\d+)/;
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Those parens serve no purpose, as you're seemingly not capturing anything, so we can be even more briefer without harming readability:

s/ /   / if m/\d+\:\d+ \d+\/\d+\/\d+/;
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