Although nowadays I'd prefer to use HTML::Parser or something similar, I think you could use a subroutine call in the replacement part (and use the /e modifier).

sub foo {
  my $bar = shift;

  if ( $CONDITION ) {
    #modify $bar if you need to
  }

  return $bar;
}

my $text =~ s{(<img.+?>)}{foo($1)}sige;
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update1: changed quantifier to +?

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