You can pick more than one value at a time out of a hash — see merlyn's code and gaal's remark.

my %hash2 = @hash{ grep /\d+_\d+/, keys %hash };
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Update: thanks to bobf for notifying me that this code makes, uh, very little sense. Here's a hash slice incantation that actually works:

my %hash2;
@hash2{ @$_ } = @hash{ @$_ } for [ grep /\d+_\d+/, keys %hash ];

# or more verbosely

my %hash2;
{
    my @key = grep /\d+_\d+/, keys %hash;
    @hash2{ @key } = @hash{ @key };
}
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