When using regular expressions it is usually a good idea to put them in a condition.

if ( my ( $raw_dt, $refs ) = /^(.*) UTC.*refs = (\d+)$/ ) {
    my $dt = $Strp->parse_datetime($raw_dt);
    ...
}
else {
    # possible error condition
}
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The greater danger of your code, apart from using uninitialized $1 and $2, is using an initialized $1 and $2 from a previous match. The capture buffer variables $1 etc are not reset to undef when a match fails so you may produce erroneous output without any outward sign.