Here's another option:

use Modern::Perl;

my $str = 'G_I1E2_GTGGAG^303CTGgacgCCCTGC_I2E3_TACA';
say for map s/[^ACGT]//gr, split 'gacg', $str;
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Output:

GGTGGAGCTG
CCCTGCTACA
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The following will produce the same printed output, but it's actually a single string with an embedded \n separating the two segments:

say $str =~ s/gacg/\n/r =~ /[ACGT\n]/g;
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